Filter materials for producing replaceable filter elements in hydraulic systems are known in many different designs (U.S. Patent Publication No. 2005/0269256 A1, U.S. Patent Publication No. 2007/0175191 A1, etc.) and comprise, for example, a nonwoven filter, preferably composed of multiple layers, having a support layer on one side, but preferably on both sides (upstream side and/or downstream side). When fluid flows through the filter material, considerable pressure differences result to some extent between the raw or dirty side and the clean side.
To be able to withstand these pressure differences and also dynamic flow forces in the unfiltrate or infiltrate (i.e., the unfiltered fluid), the filter materials, of which corresponding filter elements are made, have support layers. Such support layers or support structures are subject to an alternating pressure load during operation of the filter element. Known support structures can be formed of metal fabrics, in particular fabrics made of high-grade steel wires, which prove to be particularly stable. In the prior art, such fabrics are executed as standard fabrics (DE 600 34 247 T2), also in the manner of a plain weave, wherein the threads are always woven so as to alternate above and then again below the next thread. Known filter materials having support layers and filter layers formed in this manner do not satisfy the requirements placed on them with regard to fluid permeability, which is as high as possible, with high mechanical stability, even if, in addition to support layers and filter layers, a fluid-guiding drainage layer is provided in the form of a grid or fabric, both on the fluid upstream as well as on the fluid downstream side of the filter material, or if larger mesh sizes or coarser grids are provided, so as to obtain lower flow resistances. The latter cases result in the disadvantage of the lower resilience against alternating pressure loads during operation.